Process Capability Studies

The facilities selected for manufacture of a part are an important determinant of the cost and quality of the resulting production. If the processing equipment selected is sufficiently accurate to meet the quality target as established by drawing tolerances, reasonable costs and acceptable quality can be expected. If the processing equipment cannot consistently meet the quality target, high costs, scrap and reworked materials are inevitable outcomes.

In some companies, experienced manufacturing engineers, operators, and foremen have learned from long experience and say that "Machine lathe-15 can handle the close turning work with a close tolerance of 0.01 mm and machine mill-25 is better for the work from + 0.003 to + 0.006 mm". The processing capabilities of the facilities existing may be known to experienced people and not all plants are so fortunate as to possess this experience however. Even in the plants where a great deal is known about the capabilities of processing equipments, it is sometimes rare to have this information in such a form that it can be shared with designing engineers, planners, and methods engineers.

The knowledge about the performance capability of processing equipment is essential to the proper functioning of quality-control program. Many managers have made scientific investigation of these capabilities as a keystone of their entire product and process-control program. In so doing, they were forced to develop techniques for this investigation which were more effective for quality-control purposes than the old rule-of-thumb techniques that had prevailed for many years. One of the most useful of the techniques developed for this work is the process-capability study (PC). Process capability studies use of statistical and analytical tools. The process never display no variability, for example Mr. X-operator producing parts on Y-machine with some tolerances and the same operator on some other day may not be able to produce same range of tolerance on the same machine. Hence process variation is an inherent characteristic.

The process-capability study consists essentially of determining the capability of a single process operation, often in relation to the individual quality characteristic of a part. It is discussed in some detail below.

Objectives

After studying this unit, you should be able to

• Conduct the process capability study, and
• Calculate control limits and standard deviation.